Prep the potatoes: Wash and peel the russet potatoes. Cut into chunks.
Boil the potatoes: Place in a pot, cover with water, add salt, and boil until fork-tender, about 15-20 minutes.
Mash the potatoes: Drain and return to pot. Mash to desired consistency.
Combine ingredients: In a bowl, mix sour cream, chopped onion, melted butter, salt, and pepper. Fold into mashed potatoes.
Add cheese: Stir in grated Cheddar cheese.
Bake: Transfer to a greased baking dish, smooth the top, and bake at 350°F for 25-30 minutes until golden.
Garnish: Sprinkle with chopped chives or green onions before serving.
